ABSTRACT
Chromosomal rearrangements of the mixed lineage leukemia (MLL/KMT2A) gene leading to oncogenic MLL-fusion proteins occur in ~10% of acute leukemias and are associated with poor clinical outcomes, emphasizing the need for new treatment modalities. Inhibition of the DOT1-like histone H3K79 methyltransferase (DOT1L) is a specific therapeutic approach for such leukemias that is currently being tested in clinical trials. However, in most MLL-rearranged leukemia models responses to DOT1L inhibitors are limited. Here, we performed deep-coverage short hairpin RNA sensitizer screens in DOT1L inhibitor-treated MLL-rearranged leukemia cell lines and discovered that targeting additional nodes of MLL complexes concomitantly with DOT1L inhibition bears great potential for superior therapeutic results. Most notably, combination of a DOT1L inhibitor with an inhibitor of the MLL-Menin interaction markedly enhanced induction of differentiation and cell killing in various MLL disease models including primary leukemia cells, while sparing normal hematopoiesis and leukemias without MLL rearrangements. Gene expression analysis on human and murine leukemic cells revealed that target genes of MLL-fusion proteins and MYC were suppressed more profoundly upon combination treatment. Our findings provide a strong rationale for a novel targeted combination therapy that is expected to improve therapeutic outcomes in patients with MLL-rearranged leukemia.
Subject(s)
Drug Resistance, Neoplasm/genetics , Enzyme Inhibitors/pharmacology , Gene Rearrangement , Histone-Lysine N-Methyltransferase/genetics , Leukemia/drug therapy , Methyltransferases/metabolism , Myeloid-Lymphoid Leukemia Protein/genetics , Proto-Oncogene Proteins/antagonists & inhibitors , RNA, Small Interfering/genetics , Animals , Apoptosis , Cell Proliferation , Drug Resistance, Neoplasm/drug effects , Female , Gene Expression Regulation, Leukemic , Histone-Lysine N-Methyltransferase/metabolism , Humans , Leukemia/genetics , Leukemia/pathology , Methyltransferases/genetics , Mice , Mice, Inbred C57BL , Mice, Nude , Myeloid-Lymphoid Leukemia Protein/metabolism , Tumor Cells, CulturedABSTRACT
The transcriptional coactivator OBF-1, which interacts with Oct-1 and Oct-2 and the octamer site DNA, has been shown to be critical for development of a normal immune response and the formation of germinal centers in secondary lymphoid organs. Here we have identified the RING finger protein Siah-1 as a protein interacting specifically with OBF-1. This interaction is mediated by the C-terminal part of Siah-1 and by residues in the N-terminus of OBF-1, partly distinct from the residues required for formation of a complex with the Oct POU domains and the DNA. Interaction between Siah-1 and OBF-1 leads to downregulation of OBF-1 protein level but not mRNA, and to a corresponding reduction in octamer site-dependent transcription activation. Inhibition of the ubiquitin-proteasome pathway in B cells leads to elevated levels of OBF-1 protein. Furthermore, in immunized mice, OBF-1 protein amounts are dramatically increased in primary activated B cells, without concomitant increase in OBF-1 mRNA. These data suggest that Siah-1 is part of a novel regulatory loop controlling the level of OBF-1 protein in B cells.
